Current limiting fuses are used to interrupt current in oil filled transformers under fault current conditions. In order for the fuse to be immersible in oil, the housing must be provided with an absolute seal which will last for the life of the transformer which can be 10-20 years. The fuse must also be able to function at the full operating temperature range of the transformer which could be -30.degree. C. to a maximum of 140.degree. C.
Current limiting fuses generally consist of a cylindrical housing or tube which is capped off at each end by an end cap. A fusible element is positioned in the tube and connected to each end cap. The tube is filled with a granular filler material, such as silica sand. The filler material must have a high dielectric characteristic to prevent tracking when the fuse operates to clear the fault current otherwise the fuse would fail. It is essential, therefore, that no external contaminating material be allowed to enter the fuse. Fuses submerged in liquid material such as oil in transformers and switchgear must be provided with a seal which will prevent ingress of liquid into the fuse. One of the main problems with designing seals for under oil fuses is the difference between the temperature of the oil and the internal temperature of the fuse generated by the load current flowing through the fuse element which can cause dramatic changes in the characteristics of the materials which make up the fuse.
Fuses generally use a fiberglass/epoxy resin or a pyrex glass tube with end caps mounted on each end. The end caps are generally sealed to the tube by strong adhesives and in some instances an elastomeric part is added such as an 0-ring. The problem which has been confronted by the fuse designer is the difference in the temperature co-efficient of expansion of the metal end caps and the fiberglass tube. As the fuse is subjected to changes in temperature, the metal caps will expand and contract with respect to the fuse causing the seal to leak.